The present invention relates generally to electromagnetic components. More particularly, the present invention relates to combined electromagnetic interference (EMI) shielding and internal antennas for mobile products.
Designers of all electronic devices are under constant pressure to reduce physical size, weight and cost of the devices, and improve the electrical performance of such devices. This is particularly true of mobile devices such as cellular, personal communication system (PCS) and cordless telephones, personal digital assistants (PDAs) and other portable radio communication devices. The classification of a device as mobile and portable requires that the device be physically very small and lightweight. If the mobile or portable device is battery operated, its components must have high performance including low current drain. Since many mobile devices are intended for consumer purchase, they and their components must be low cost in accordance with the low profit margins on consumer devices. This low cost requirement also dictates that the electronic device be simple and inexpensive to manufacture. Manufacturing should be automated, with as few steps as possible, and the components and the steps to assemble them should not introduce defects which can compromise production yields.
Radio communication devices typically required shielding between sources of electromagnetic interference (EMI) and sensitive circuitry. Conventionally, such devices include one or more printed circuit boards contained within a housing. EMI shields isolate the circuitry from EMI source so that the energy emitted by the EMI source does not affect the circuitry. Known deleterious effects in the absence of suitable EMI shielding include unwanted frequency shifts, oscillations, intermodulation distortion, overloading of receive circuits during transmission, etc. EMI shields are also used to reduce cross talk between different subsystems, such as the receiver and transmitter in a transceiver.
FIG. 1 is an isometric view of a prior art electronic device 100 including internal antennas combined with electromagnetic interference shields. The device 100 includes a printed circuit board 102 with electronic components 104 mounted thereon. An EMI shield 106, also referred to as a can, is positioned over some or all of the components 104. A plastic support frame 108 is positioned over the EMI shield 106. One or more antennas 110, 112 are disposed on the plastic support frame 108.
The electronic components 104 may perform any of the digital, analog, or RF functions of the electronic device 100. The EMI shield 106 forms a ground plane for the antennas 110, 112. Thus, the EMI shield 106 is in electrical contact with the printed circuit board 102 so that the EMI shield 106 is electrically grounded. The antennas 110, 112 must electrically contact the printed circuit board 102 at a feed point driven by receive or transmit circuitry of the printed circuit board 102.
The conventional solution shown in FIG. 1 has been effective but there is still room for improvement. For example, the illustrated solution requires additional space in the plane of the printed circuit board 102 to mount both the EMI shield 106 and the plastic support frame 108. To minimize manufacturing costs, these components must be compatible with surface mount manufacturing processes. Proper tolerances must be maintained in spacing these components from other components such as the electronic components 104. Since electrical contacts must be made when placing the EMI shield 106 and the antennas 110, 112, these components must be reliably soldered to the printed circuit board 102. Proper vertical tolerances above the plane of the printed circuit board 102 must be maintained as well. The antenna resonant frequency is dependent on the vertical spacing between the radiating antennas 110, 112 and the ground plane formed by the EMI shield 106. However, vertical tolerances are particularly difficult to control in a surface mount assembly process.
Accordingly, there is a need for an improved antenna and EMI shield method and apparatus.